This invention relates to the field of regulation of gene expression.
Animal cells undergo replicative senescence--age-dependent decrements in function and integrity. One aspect of replicative senescence is that cells have a finite replicative life-span. After undergoing a limited number of mitotic divisions, they can no longer divide. Upon replicative senescence, cells stably arrest growth with a G1 DNA content, irreversibly losing the ability to enter S phase in response to physiologic mitogens. Another aspect of replicative senescence is that senescent cells acquire altered functions, resembling terminally differentiated cells. A third aspect of replicative senescence is that senescent cells acquire resistance to apoptotic cell death.
Senescent cells exhibit altered gene expression compared with pre-senescent cells. In particular, several genes increase expression. For example, senescent human fibroblasts show a marked increase in interstitial collagenase mRNA and protein. Burke et al., Exp. Gerontol., (1994) 29:37-53. Senescent cells also exhibit increased expression of stromelysin, the adhesion molecule ICAM, and certain inflammatory mediators, such as IL-1. Other genes, such as certain mitogen inducible genes and IL-6, are repressed in senescent cells. Altered gene expression manifests itself at the tissue and organismic level.
For example, collagenase breaks down collagen, particularly, the fibrillar collagens (types I, II and III). The extracellular matrix is made of a complex network of proteins and proteoglycans, including collagen, that maintain the integrity of tissues. J. D'Armiento et al., Cell, (1992) 71:955-961. The turnover of these molecules is effected by, among other things, matrix metalloproteinases such as interstitial collagenase. Interstitial collagenase has been implicated in osteoarthritis, periodontal lesions, bullous skin lesions, cancer invasion and a variety of liver and lung diseases. For example, injection of collagenase into the joints of mice results in lesions similar to osteoarthritis. P. M. van der Kraan, J. Exp. Path., (1990) 71:19-31. Emphysema is characterized by disruption of the alveolar walls and coalescence of the alveolar spaces. Mice made transgenic for a construct that expresses the collagenase gene in lung cells exhibited these symptoms. D'Armiento et al., supra. Epithelial organs, including the skin, have an outer epidermal layer, a basement membrane and a layer of dermis comprising a stroma that includes fibroblasts. Skin, in particular, is composed of stratified, squamous, keratinized epithelium, called the epidermis. Aged skin contains a greater number of senescent cells than young skin. Aged skin tends to be wrinkled, dry and scaly. Mice made transgenic for a construct that expresses the collagenase gene in skin took on characteristics of aged skin: Their skin was dry, wrinkled and scaly, and exhibited acanthosis, hyperkeratosis and epidermal hyperplasia. Also, the transgenic mice demonstrated increased susceptibility to tumorigenesis. J. D'Armiento et al., Molec. and Cell Biol., (1995) 15:5732-39.
Certain premature aging syndromes, such as Werner syndrome, are characterized by premature cellular senescence. Persons with Werner syndrome show hair and dermal thinning, atherosclerosis, osteoporosis and increased incidence of cancer.
Replicative senescence and its regulation are an increasingly active area of study. Genetic markers of senescence and methods of expressing genes in senescent cells or altering expression of genes in senescent cells would constitute advances in the art.